A radiographic phosphor panel contains a layer of phosphor, a crystalline material which responds to X-radiation on an image-wise basis. Like many other crystalline materials, radiographic phosphors have a crystal matrix which allows for the replacement of some atoms by other similar atoms, but does not readily accept other atoms or moieties. Radiographic phosphor panels can be classified, based upon their phosphors, as prompt emission panels and image storage panels.
Intensifying screens are the most common prompt emission panels. Intensifying panels are used to generate visible or near visible (i.e., UV or IR) light upon exposure of the intensifying panel to X-radiation. A sheet of photographic film is positioned to intercept the visible light generated and commonly is pressed against the intensifying panel within a light-tight cassette. Other prompt emission panels operate similarly, but in place of the photographic film have some other means for visualizing the X-radiation.
Storage panels have storage phosphors, that have the capability of storing latent X-ray images for later release, apparently by locally trapping electron-hole pairs created by incident X-rays. Storage phosphors are distinguishable from the phosphors used in X-ray intensifying or conversion screens. In the latter, a latent image is not stored and X-radiation causes the immediate release of visible or near visible light from irradiated phosphor crystals.
X-ray storage phosphors and image storage panels are characterized by a number of properties including sensitivity, resolution and noise. It is generally desired that the sensitivity or the photostimulated luminescence of the panel be made as high possible to decrease the exposure dose for the patient while maintaining high image quality. Additionally, X-ray storage phosphors and image storage panels are characterized by several other properties including dark-decay; the ability of the phosphor or panel to retain the signal (image) over time, and erasability; the ease with which the image may be removed so that a new image can be "written" on the screen.
Radiation image storage panels are used in computed radiography. The panel is first exposed to X-radiation to create a latent image. The panel is then stimulated with longer wavelength radiation, resulting in the emission of radiation at a third wavelength. Typically a laser having a red or infrared beam is scanned over the panel, resulting in the emission of green or blue radiation. The emitted light is collected and the resulting signal is processed electronically to produce a final image.
In the aforementioned radiation image recording and reproducing method, after the patient is exposed and the image is detected and read, it is necessary to remove or "erase" the image from the panel so that the panel can be reused to collect further images. If the image is not completely erased a problem arises in that subsequent images are degraded by excessive noise and/or by "ghost" images. Japanese Patent Provisional Publication No. 56(1980)-11392 discloses a method for erasing the stored energy which comprises exposing the panel to light having a wavelength within the region of stimulation wavelength of the stimulable phosphor. The erase step is performed prior to the panels reuse. The erasability of a phosphor or panel can be expressed as the "erase fraction". This is a dimensionless value that represents, for specified conditions, the residual luminescence of a storage phosphor after erasure, in proportion to the stimulated luminescence of the phosphor. It is desired that a stimulable phosphor employed in a storage panel have high erasability, that is, that the decay of stimulated emission is as fast as possible upon exposure to light having a wavelength within or near the region of stimulation.
The phosphors of the present invention are derived from the family of barium fluorohalide storage phosphors. Barium fluorohalide storage phosphors are commercially used as X-ray sensors and/or image storage devices in computed (or digital) radiography. The halide utilized is typically bromide, or a combination of bromide and iodide with europium as the activator, e.g., BaFBr.sub.1-z I.sub.z :Eu.
Alkaline earth metal fluoro-halide storage phosphors are described in a number of patent publications. European Patent Application No. 0142734 A1 teaches a phosphor described by the formula: EQU BaF(Br.sub.1-z I.sub.z):yEu.sup.2+
This application states that . . . "the phosphor containing three kinds of elements of fluorine, bromine and iodine as halogen which is a host component of the phosphor is prominently enhanced in the luminance of stimulated emission. The radiation image recording and reproducing method employing said stimulable phosphor can be remarkably enhanced in the sensitivity."
U.S. Pat. No. 4,505,989, to Umemoto et al., discloses a storage phosphor like the phosphor of formula (I), that is coactivated with a transition metal, and has improved erasability and photostimulated luminescence. The patent indicates improvements in relative erasing time of 1 to 30 percent upon inclusion of a transition metal into the phosphor of formula (I). The phosphor is prepared by firing precursors:
"[T]he mixture of the starting materials for the phosphor is placed in a heat-resistant container such as a quartz boat, an alumina crucible or a quartz crucible, and fired in an electric furnace. The temperature for the firing suitably ranges from 600.degree. C. to 1000.degree. C. The firing period is determined depending upon the amount of the mixture of the starting materials charged into the heat resistant container, the firing temperature, etc., and generally ranges from 0.5 to 12 hours. As the firing atmosphere, there can be employed a weak reducing atmosphere such as a nitrogen gas atmosphere containing a small amount of hydrogen gas or a carbon dioxide gas atmosphere containing carbon monoxide gas. PA1 The product obtained by firing conducted under the above-mentioned condition is taken out of the furnace, allowed to stand for cooling, and pulverized. The pulverized product may be again placed in the heat-resistant container and fired in the electric furnace. In the second firing, the temperature of the firing suitably ranges from 500.degree. to 800.degree. C. and the firing period suitably ranges from 0.5 to 12 hours. For carrying out the second firing, there can be employed an inert atmosphere such as a nitrogen gas atmosphere or an argon gas atmosphere, as well as the above-mentioned weak reducing atmosphere." (column 6) PA1 "a europium-doped barium fluorohalide photostimulable phosphor of the formula: EQU Ba.sub.1-x Eu.sub.x FBr.sub.1-2y O.sub.y .O slashed..sub.y PA1 wherein .O slashed. represents an anion vacancy, x is about 0.001 to about 0.05, and y is about 0.0001 to about 0.01; the amount of oxygen vacancies effective to substantially increase the stored photostimulable energy, compared to a non-oxygen-treated phosphor."
The phosphor is then processed and coated according to various procedures well known in the art to form an image storage panel.
U.S. Pat. No. 5,227,254, to Brixner et al., discloses:
The phosphor of this patent is obtained by heating a phosphor precursor at about 900.degree. C. in a stream of nitrogen containing 1 to 2% oxygen. The patent reports a significant increase in photostimulated luminescence over that of a material prepared by the "conventional" method.
U.S. patent application Ser. No. 08/157,582, filed Nov. 24, 1993 teaches a phosphor which is the
Product of firing precursors including an oxo-sulfur reducing agent. U.S. patent application Ser. No. 08/157,583, filed Nov. 24, 1993 teaches an alkaline earth metal fluorobromoiodide storage phosphor that includes potassium. These two patent applications are hereby incorporated herein by reference.
It would be highly desirable to provide a storage phosphor, preparation method, and image storage panel having improved erasability while maintaining high photostimulated luminescence.